The overall objectives of this research are to understand the optical properties of human skin, to investigate responses of normal and abnormal human skin and of peripheral blood lymphocytes to nonionizing electromagnetic radiation with and without photosensitizers, and to relate these two broad areas qualitatively and quantitatively. We will utilize this data to mathematically model cutaneous and cellular optical properties. New exposure sources and the capability to quantify erythema permit extending measurement of the human erythema action spectra and dose-response curves to wavelengths longer than 315 nm with and without photosensitizers, and to wavelengths shorter than 250 nm. This quantified phototoxic endpoint also permits the first precise studies of waveband interactions, and the effects of temperature and infrared on photobiologic responses of the intact skin of living man. The action spectrum and dose-yield plots for photochemical production of previtamin D3 and its photoisomers will be correlated with spectral photon densities in skin of widely differing skin pigmentation levels. The lymphocyte will be used as an in vitro model to compare with and contrast with the in vivo cutaneous dose and response parameters of phototoxicity quantified by the proposed work.